Ruth Montano, 55, had ordered mentally disturbed son David Elias, 31, to leave her East Harlem apartment Saturday afternoon after he got into an argument with her and her 32-year-old son, law-enforcement sources said yesterday.

That’s when Elias allegedly flew into a deadly rage, the sources said.

“They wanted [Elias] to leave, and he flipped out,” one of the sources said.

Elias allegedly grabbed a knife and stabbed his older brother in the head and torso — and when his mother tried to intervene, he stabbed her in the chest.

“The mother tried to stop him from stabbing the brother,” the law-enforcement source said. “That’s when she got killed.”

Elias then set a small fire inside his mother’s 10th-floor apartment on Park Avenue near 135th Street and fled with his face and sneakers covered in blood, cops and witnesses said.

Within a half-hour, firefighters were called to the scene and found the mom and older son as they extinguished the blaze. Police investigators then quickly zeroed in on Elias as a suspect and obtained his description, cops said.

The eagle-eyed cops asked Elias his name and arrested him after he identified himself.

Montano was pronounced dead at Harlem Hospital. Her stabbed son was in critical condition when he was admitted to the hospital Saturday, but his condition improved to stable yesterday.

Neighbor Anthony Watts, 16, told The Post he was playing basketball nearby when he saw a shoe fly out of a 10th-floor apartment and heard a man scream, “Help me! Help me, please! My brother’s trying to kill me!”

When Watts rushed into the building, he saw Elias and asked him what had happened. The blood-soaked man answered, “You’ll understand. You’ll know what I’m talking about,” before calmly walking out.

Elias doesn’t appear to have a criminal history, but was involved in two domestic incidents with his brother — in 2009 and 2011 — in which he appeared to be the aggressor, cops said.

Elias has been treated for mental illness in the past and did not live with his mother, law-enforcement sources said.

Investigators found several knives in Montano’s apartment but had not yet determined whether one was the murder weapon, cops said.

Elias has been charged with murder, attempted murder and arson.

He was awaiting arraignment in Manhattan Criminal Court last night when he was rushed to Beth Israel emergency room for unknown reasons.